The use of polymers as a rheology modifier (or viscosity modifier) or as a dispersant in an oil of lubricating viscosity is well known. Typically polymers include a polymethacrylate with physical properties that have high and low temperature viscometrics as well as shear stability. For typical linear polymers these properties are related to the polymer's molecular weight. Therefore the useful molecular weight range for lubricating compositions is limited. Polymer molecular weights chosen optimize shear performance, for example, may then result in unacceptable low temperature viscometrics or render reduced fuel economy.
In an attempt to overcome the limitations of linear polymers, star polymers have been disclosed in International publication WO 06/47398, WO 06/47393, WO96/23012 A1 and European patent applications EP 979 834 A2 and EP 936 225 A1. The star polymers of WO 06/47398, WO 06/47393 are prepared by RAFT, ATRP or nitroxide mediated stable free-radical polymerisation; whereas WO96/23012 and EP 936 225 disclose star polymers prepared from anionic polymerisation techniques.
The processing of the star polymers is complex. The star polymer disclosures describe making polymers with arm-first, core-first or arm-core-arm approaches. In addition anionic polymerisation processes require more complex processing. For instance, the process requires highly pure solvents and an inert atmosphere substantially free of water, and typically performed at sub-ambient temperatures.
It would be desirable to have a polymer suitable for use in lubricating compositions with at least one of acceptable viscosity index improving characteristics, acceptable cleanliness, acceptable shear stability, acceptable viscosity index per thickening efficiency and acceptable dispersant properties. Further it would be desirable to have a polymer prepared with less complex processing than may be used for conventional polymers, e.g., without the use of special or non-commercial catalysts/initiators, purified solvents, or sub-ambient temperatures. The present invention provides a polymer with such properties.